1. Field of Invention
The present invention relates to a circuit for measuring time interval, and more particularly, to a circuit for measuring a time interval using ring oscillator scheme.
2. Description of the Related Art
In signal processing field, a time interval corresponding to the pulse width of a pulse signal, or a time interval between two digital pulse edges is usually needed. For example, when measuring distance with laser, calculating a time interval between the start digital signal when emitting laser pulse and the stop digital signal when receiving the reflected laser signal from the object, and multiplying with light speed, a distance between the origin and the object is obtained. Generally speaking, the time interval described above is substantially short, usually measured in the order of nanosecond (ns). Therefore, it is desired to measure a time interval with high precision.
Referring to FIG. 1, it illustrates a circuit for measuring time interval. In FIG. 1, the time interval measuring circuit is named as propagation delay time-to-digital converter (TDC), including a plurality of propagation delay units 101 connected in series, an assembled with an encoder 103. Wherein the propagation delay time of the propagation delay unit is used as a unit time. When a pulse signal V1 is fed to the starting terminal Start, the propagation delay TDC begins to propagate signal V1, and subsequently changes the state of each propagation delay unit. After a specific number of propagation delay unit times, a V2 signal is received and signals of each of the propagation delay units are transferred to the encoder, where an amount of propagation delay unit is readout, such that the time interval Tin corresponding to input signals V1 and V2 is measured. This propagation delay TDC provides high precision (tens of ps), yet is limited in expandability and measuring range. If to increase measuring range, an amount of propagation delay units is increased, yet complexity of circuit layout is increase along, which is hard to implement.
Referring to FIG. 2, it illustrates a time interval measuring circuit named as linear time-to-digital converter (TDC), using pulse-shrinking delay elements, e.g. PDE 201 along with delay locked loop 203. When the input pulse signal Vin is fed to the first PDE 201, each pulse-shrinking delay element serves to reduce a constant pulse width from the pulse signal Vin, the last stage of the delay unit adjusts the bias signal Vbias such that time span shortened by the delay unit is changed till pulse width of the input signal Vin is equal to zero at the output of the last stage. Multiplying the adjusted time span with the amount of the delay units under designed, then the time interval corresponding to the pulse width of the pulse signal is obtained. The linear TDC is more stable when the temperature varies, yet the bias voltage Vbias has to be calibrated during measurement for different pulse widths of the pulse signal Vin, where measurement precision and range are limited by an amount of delay units and bias voltage adjusting range.
Referring to FIG. 3, it illustrates another conventional time interval measuring circuit. In FIG. 3, the circuit is named as cyclic time-to-digital converter (TDC), using pulse-shrinking delay element 301 for constantly reducing pulse width of the input pulse signal Vin, and feeding back the pulse signal Vin via the delay line circuit 303 till the pulse width of the pulse signal is reduced to zero. Wherein the delay line circuit 303 has a plurality of delay units 311, for example. When the pulse width of the pulse signal becomes zero, the counter 305 accumulates the times that pulse signal Vin passes through the delay line circuit for calculating the time interval corresponding to the pulse width of the pulse signal Vin. The cyclic TDC improves the calibrating procedure of the linear TDC as mentioned above, yet the resulting value will receive delay by multiple times for repeatedly inputting signal to the pulse-shrinking delay element. The measuring range of the cyclic TDC is limited by the maximum delay time featured with the delay line circuit 303. When attempting to increase the measuring range, the complexity of circuit structure is increased as well.